Guide — Labs & Diagnostics
Complete Blood Count (CBC) for the Respiratory Therapist
The CBC sits next to the ABG on almost every respiratory patient. This guide reads it the way an RT does — the white count and differential for infection, hemoglobin and hematocrit for oxygen-carrying capacity, and the platelet count before any invasive procedure.
8 min read · Labs & Diagnostics
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Overview
The complete blood count is one of the most frequently ordered labs on a respiratory patient, and the respiratory therapist reads it for three distinct reasons. The white blood cell count and differential help judge whether a deterioration is infectious. The hemoglobin and hematocrit reveal how much oxygen the blood can actually carry — a number that pulse oximetry cannot show. And the platelet count gates whether it is safe to place an arterial line, perform a bronchoscopy with biopsy, or insert a chest tube. Reading the CBC alongside the arterial blood gas turns a saturation number into a true picture of oxygen delivery.
Key Concepts
A CBC has three pillars the RT cares about: the white cells (is there infection or stress?), the red cell mass — hemoglobin and hematocrit — (can the blood carry oxygen?), and the platelets (can the patient tolerate a procedure?). None of them is interpreted in isolation; each is correlated with the clinical picture and the ABG.
- White blood cells:normal 4,500–11,000/µL (4.5–11 × 10⁹/L). Leukocytosis points to bacterial infection, inflammation, physiologic stress, or corticosteroids. Leukopenia (<4,500) suggests overwhelming sepsis, marrow suppression, or some viral infections, and implies immunocompromise.
- Hemoglobin & hematocrit:Hgb 13.5–17.5 g/dL (male), 12.0–16.0 g/dL (female); Hct runs roughly three times the Hgb (about 41–53% male, 36–46% female).
- Platelets:normal 150,000–400,000/µL. They govern bleeding risk and procedural readiness.
The white count and differential
A raised white count is sensitive but not specific: it rises with bacterial infection, inflammation, and physiologic stress, and corticosteroids alone can push it up without any infection. The differential adds resolution. Neutrophils are the first bacterial responders, and a left shift — an increased number of bands, the immature neutrophils — classically marks acute bacterial infection. For the RT phenotyping asthma, the eosinophil count is the line to watch: eosinophilia above 500/µL or 5% supports an allergic or eosinophilic phenotype.
| Cell line | Typical fraction | What it suggests |
|---|---|---|
| Neutrophils | 40–70% | First responders to bacterial infection; a left shift (increased bands) signals acute bacterial infection. |
| Lymphocytes | 20–40% | Predominate in viral infection; relative changes help separate viral from bacterial pictures. |
| Eosinophils | 1–4% | Eosinophilia (>500/µL or >5%) points to asthma, allergy, ABPA, eosinophilic pneumonia, or parasitic disease; helps phenotype asthma. |
| Monocytes / Basophils | Minor | Smaller fractions; less central to routine respiratory interpretation. |
Hemoglobin, hematocrit, and oxygen-carrying capacity
This is the core respiratory point. Each gram of fully saturated hemoglobin carries 1.34 mL of O₂, so the arterial oxygen content is CaO₂ = (1.34 × Hgb × SaO₂) + (0.003 × PaO₂). The dissolved term is tiny; hemoglobin does almost all the work. Anemia therefore lowers CaO₂ even when SpO₂ and PaO₂ are perfectly normal — a “good” pulse-ox reading does not guarantee adequate oxygen delivery, because pulse oximetry cannot detect anemia. When a patient with a normal saturation is dyspneic or desaturates on exertion, the hemoglobin is the value to check.
At the other end, secondary or appropriate polycythemia is the marrow’s response to chronic hypoxemia — COPD, obstructive sleep apnea, living at high altitude, and chronic hypoxemic lung disease. A rising hematocrit is a useful clue that hypoxemia has been present for a long time. But once the hematocrit climbs above roughly 55–60%, the increased blood viscosity impairs flow and oxygen delivery. Distinguish relative polycythemia (hemoconcentration from dehydration) from absolute, and secondary polycythemia from primary disease (polycythemia vera).
Platelets before invasive procedures
The platelet count sets the floor for invasive respiratory work. Thrombocytopenia (<150,000/µL) raises bleeding risk, and a count near 50,000/µL is a common practical threshold below which procedures — arterial line placement, bronchoscopy with biopsy, chest tube insertion — are deferred or the count is corrected first. A count under 20,000/µL carries a risk of spontaneous bleeding. Verify platelets and coagulation status before any arterial or pleural procedure, and follow your institution’s policy.
What the RT does with it
- SpO₂ fine but dyspneic or desaturating on exertion:check the hemoglobin — anemia lowers oxygen delivery without changing the saturation.
- Suspected infective exacerbation: correlate leukocytosis and a left shift with a suspected infectious COPD exacerbation or pneumonia to support the clinical picture.
- Before any arterial or pleural procedure: verify the platelet count and coagulation status before drawing an ABG line, assisting with bronchoscopy and biopsy, or placing a chest tube.
- Rising hematocrit: read a secondary polycythemia as a marker of chronic hypoxemia and factor blood viscosity into your assessment when it climbs high.
Common Pitfalls
- Trusting a normal SpO₂ as proof of adequate delivery: the saturation ignores hemoglobin entirely, so anemia can leave delivery low while the pulse-ox reads reassuringly high.
- Confusing relative with absolute polycythemia: dehydration concentrates the blood and mimics a true rise in red cell mass; the management is different.
- Overlooking eosinophilia when phenotyping asthma: the eosinophil count is a quick clue to an allergic or eosinophilic phenotype that is easy to skip past.
- Treating the WBC in isolation: corticosteroids alone raise the white count, so a high number without a left shift or clinical context can mislead.
Board Exam Pearls
- Know the CaO₂ equation:CaO₂ = (1.34 × Hgb × SaO₂) + (0.003 × PaO₂), and remember the 1.34 mL O₂ per gram of hemoglobin constant.
- Left shift = bands: increased immature neutrophils point to acute bacterial infection.
- Secondary polycythemia is a clue to chronic hypoxemia: tie a high hematocrit back to COPD, OSA, or high-altitude living.
- The ~50,000/µL platelet threshold: below it, invasive procedures are typically deferred or the count corrected first.
FAQ
Why doesn't a normal SpO₂ rule out impaired oxygen delivery?
Pulse oximetry reports the percentage of available hemoglobin that is saturated — it says nothing about how much hemoglobin is present. Arterial oxygen content, CaO₂ = (1.34 × Hgb × SaO₂) + (0.003 × PaO₂), is driven mostly by the hemoglobin term, so a severely anemic patient can read SpO₂ 99% and still have a low CaO₂ and inadequate oxygen delivery. A reassuring pulse-ox number does not guarantee the tissues are getting enough oxygen; when a patient is dyspneic or desaturates on exertion with a normal SpO₂ at rest, check the hemoglobin.
What is a left shift, and what does it suggest?
A left shift is an increased number of bands — immature neutrophils — appearing in the differential as the marrow pushes out cells to meet demand. It classically signals an acute bacterial infection. For the RT, a left shift alongside leukocytosis supports an infectious cause for a COPD exacerbation or pneumonia and helps frame the urgency of antibiotics and pulmonary toileting.
Why does chronic COPD or OSA raise the hematocrit?
Chronic hypoxemia stimulates erythropoietin, and the marrow responds by making more red cells — secondary or appropriate polycythemia. Conditions such as COPD, obstructive sleep apnea, chronic hypoxemic lung disease, and living at high altitude all drive it. A rising hematocrit is therefore a clue to longstanding hypoxemia, but once it climbs above roughly 55–60% the increased blood viscosity itself impairs flow and oxygen delivery.
What platelet count is generally needed before an invasive respiratory procedure?
A platelet count near 50,000/µL is a common practical threshold: below it, procedures like arterial line placement, bronchoscopy with biopsy, or chest tube insertion are often deferred or the count is corrected first. A count under 20,000/µL carries a risk of spontaneous bleeding. Always verify platelets and coagulation status before any arterial or pleural procedure, and follow your institution's specific policy.
Go deeper
Match each derangement to its cause and its bedside implication.
See the CBC abnormalities chart →Related Resources
Sources
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021. Oxygen and carbon dioxide transport; hematologic assessment.
- National Board for Respiratory Care. Therapist Multiple-Choice Examination Detailed Content Outline. NBRC; 2024. Patient data evaluation and recommendations.